Accounts are not deleted from Gaia, they are kept in case the history of that account is needed for an investigation.

if you don't want to use it any more just abandon it. change the username to a bunch of random
letters (to free it up for others to use), let every one on your friends list know your abandoning it,
and change the password to a bunch of random letters.

Although accounts are never deleted you can request it be closed.
Delete anything on the account you don't wish to remain and make the request via the contact us link at the bottom of the page.
This action is not reversable, so if you'll want the account again later its best to just change the password to something complicated and then stop using it.
that way you can always do a forgot password when you want to come back to it.